The present invention relates generally to the field of optical signal processing apparatus, and more specifically to a differentiating spatial light modulator of simplified construction and improved performance.
Two-dimensional spatial light modulators are devices which allow control of an optical wavefront for processing or imaging operations. These devices, often referred to as light valves in the literature, have potential for application in large screen display systems as well as in optical data processing systems, including missile guidance and robotic vision systems. Listed below are several articles which describe their construction and operation.
1. "A Fast Silicon Photoconductor-Based Liquid Crystal Light Value", P. O. Braatz, K. Chow, U. Efron, J. Grinberg and M. J. Little, IEEE International Electron Devices Meeting, pp. 540-543, 1979.
2. "Oblique-cut LiN.sub.b O.sub.3 Microchannel Spatial Light Modulator", C. Warde and J. I. Thakara, Optics Letters, Vol. 7, No. 7, July 1982.
3. "A First-Order Model of a Photo-Activated Liquid Crystal Light Valve", J. D. Michaelson, SPIE Vol. 218, Devices and Systems For Optical Signal Processing, 1980.
4. "LiNbO.sub.3 and LiTaO.sub.3 Microchannel Spatial Light Modulators", C. Warde, A. M. Weiss and A. D. Fisher, SPIE Vol 218, Devices and Systems for Optical Signal Processing, 1980.
5. "Silicon Liquid Crystal Light Valves: Status and Issues", U. Efron, P. O. Braatz, M. J. Little, R. N. Schwartz and J. Grinberg, Proc. SPIE Vol. 388, Jan. 1983.
6. "Applications of Priz Light Modulator", D. Casasent, F. Caimi, M. Petron and A. Khomenko, Applied Optics, Vol. 21., No. 21, November 1982, pp. 3846-3854.
7. PLZT Color Displays, G. Haertling, SID 84 Digest, pp. 137-140.
Spatial light modulators often comprise a photosensitive semiconductor substrate (photodiode), a light blocking layer, a dielectric mirror and an electro-optic crystal (which may be a liquid crystal), arranged in a sandwich-like composite structure, and having a voltage applied thereacross. A control (write) illumination impinges on the face of the photosensitive semiconductor while an output (read) illumination makes a double pass through the electro-optic crystal.
The photosensitive semiconductor responds to intensity variations in the control illumination impinging thereon. In the dark, most of the voltage applied across the composite structure appears across the reverse-biased photodiode. The write beam, however, excites carriers in the silicon, which are driven by the internal field to the Si/electro-optic crystal interface. The voltage across the silicon decreases, while the voltage across the electro-optic crystal increases. The read illumination passes through the electro-optic crystal, is reflected off of the dielectric mirror, and again passes through the electro-optic crystal before emerging from the device. Since the diffraction efficiency of the electro-optic crystal is a function of the voltage applied thereacross, (which is a function of the intensity of the write illumination), optical control of the output (read) illumination is achieved.
It is known that in the recognition of images, about ninety percent of the information content resides at the edges of the image, where abrupt changes in light intensity occur. A form of spatial light modulator called the Priz modulator has been designed which modulates light by the transverse (rather than the longitudinal) electro-optic effect. Thus, only the edge contour of an object appears and edge enhancement is achieved.
The operation of the Priz spatial light modulator involves the use of a BSO (bismuth silicon oxide, Bi.sub.12 Si O.sub.20) crystal which is cut such that the device modulates light by the transverse rather than the longitudinal electro-optic effect. The spatially varying light distribution is still incident on the crystal's large faces collinear with the applied electric field direction, and the spatially varying charge layer parallel to the crystal's large faces is still induced. However, the transverse component of this field is what is used to provide the spatial modulation of the incident light.
In the Priz spatial light modulator, the photoconductor and electro-optic crystal are one and the same BSO element. This can have the disadvantage of erasure of the image by the readout light. Alternatively, cyclic operation requires an optical erasure operation which removes the previously recorded image. Moreover, the BSO crystal has a somewhat limited wavelength range of operation.